Fluid therapy

During flow phase resuscitation, fluid therapy maintains intravascular volume in patients where initial management has already demonstrated success (i.e. ongoing losses as in hemorrhage). This maintains the preload levels necessary to support cardiac output and systemic O 2 delivery. Crystalloids and/or colloids are used for this objective. Fluid therapy also repletes the intravascular fluid lost to the extravascular compartment which characterizes the physiological adaptation to hypotension. Given the characteristics of extravascular fluid, crystalloids (i.e. Ringer's lactate) are preferred.

Each fluid prescription must be followed by reassessment to determine whether the therapeutic objective is being met. Where cardiogenic causes are suspected, the use of fluid to increase cardiac output is undertaken with particular caution with fluid challenges given in small steps (i.e. 50-100 ml colloid). In contrast, fluid challenges after hemorrhage are typically more aggressive (250-500 ml).

Debate persists as to the 'best' fluid for both emergent resuscitation and ongoing maintenance of intravascular volume in critically ill patients. With emergency resuscitation of intravascular volume, i.e. in hypovolemic and vasodilated septic shock, we recommend a balanced combination of colloids (e.g. albumin, Pentastarch)

and isotonic crystalloids. Some practitioners recommend hypertonic saline as part of the early management of hypovolemic shock. Smaller doses of colloids and hypertonic solutions increase intravascular volume faster than larger volumes of isotonic crystalloids. They also facilitate movement of interstitial fluid into the circulation, making them more efficient resuscitation agents. However, attention should be paid to restoring extravascular volume during subsequent flow phase resuscitation. The most commonly used crystalloid solutions are Ringer's lactate and normal saline. Excessive use of normal saline may result in a hyperchloremic metabolic acidosis. Electrolyte solutions are the most commonly used fluids for maintenance therapy, although colloids are recommended if the colloid osmotic pressure remains excessively depressed.